<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        .box {
           padding: 10px;
           padding-bottom: 0;
           padding-right: 0;
           display: flex;
           flex-direction: column;
           height: 800px;
           width: calc(100vw - 20px);
           background-color: #000;
         }
         .content {
           display: flex;
           margin-bottom: 10px;
           flex: 1
         }
         .b {
           flex: 1;
           margin-right: 10px;
           background-color: red;
         }
       </style>
</head>
<body>
    <div class="box">
        <div class="content">
          <div class="b">1</div>
          <div class="b">2</div>
          <div class="b">3</div>
        </div>
        <div class="content">
          <div class="b">4</div>
          <div class="b">5</div>
          <div class="b">6</div>
        </div>
        <div class="content">
          <div class="b">7</div>
          <div class="b">8</div>
          <div class="b">9</div>
        </div>
      <div>
</body>
</html>